(1) Field of the Invention
The present invention relates to a method of fabricating a spar for a blade, to a method of fabricating the blade, and the resulting blade.
(2) Description of Related Art
A rotorcraft has at least one engine that drives a main rotor in rotation in order to provide the rotorcraft with lift and possibly also propulsion. The main rotor has a hub carrying a plurality of blades.
Each blade conventionally comprises at least one spar extending spanwise from the root of the blade.
While they are rotating, the blades are subjected to a force torsor. The blades are subjected to centrifugal force and also to multiple forces and bending and twisting moments due to the movements of the blade, in particular due to its flapping movements and its lead-lag movements.
Consequently, a particular function of the spar is to transmit the forces to which the blade is subjected to the hub, and in particular to transmit centrifugal force.
A first type of blade has a spar arranged at the leading edge of the blade. The spar extends from the root of the blade along the span of the blade. That type of spar is referred to below for convenience as a “leading-edge” spar.
Such a leading-edge spar also contributes to static chord balancing of the blade.
Furthermore, the leading-edge spar tends to improve ability to withstand damage caused as a result of an object impacting against the leading edge.
A second type of blade has a spar referred to as a “distributed” spar. Such a distributed spar consists in a leading-edge spar that is extended in its running portion by two portions that are arranged flat respectively on the pressure side and on the suction side of the blade. In its running portion, a distributed spar thus has cross-sections of C-shape in transverse planes. Each transverse plane is substantially parallel to the axis of rotation of the rotor and perpendicular to the span of the blade.
The distributed spar thus has a solid leading edge. This leading edge is extended by a top flap extending over the suction side of the blade and a bottom flap extending over the pressure side of the blade.
In addition to having the advantages of a leading-edge spar, a distributed spar tends to make the blade stiffer in flapping.
Furthermore, a distributed spar tends to optimize the strength of the blade by imparting favorable strength to the blade in the event of local damage.
Nevertheless, it is difficult to connect a blade having a distributed spar to a rotor hub.
In order to be connected to a hub, the distributed spar of a blade may for example be wound at its root end around at least one vertical axis that is substantially parallel to the axis of rotation of the rotor. The blade is then fastened to the rotor by inserting a pin extending along such a vertical axis.
Nevertheless, fabricating such a distributed spar that is wound around at least one vertical pin can be difficult.
In a first step, an operator fabricates tape by impregnating resin into glass fiber roving arranged side by side and calendared to have the dimensions of the desired tape. The tape is assembled with a separator film and then wound on a reel or “roll”.
Prior to being polymerized or consolidated, the roving tape used for fabricating blades presents very good capacity for deformation. This deformation capacity enables an operator to obtain complex variations in shape by smoothing manually.
In a second step, the operator builds individual hanks from segments of a roving tape. Each hank then presents a plurality of layers of roving type.
Each individual hank is in the form of a long loop closely wound around a root wedge, the hank having sections that are substantially rectangular. Each hank has a C-shape in a plane that is substantially orthogonal to the axis of rotation of the rotor. Thus, each hank has two lateral strands connected to an end wall juxtaposed against the root wedge. Such a root wedge is an elongate part made of composite materials that is for receiving a fastener bushing.
In a third step, the operator drapes a skin over a first half-shell of a mold in order to embody the pressure side of the blade, and over a second half-shell of the mold in order to embody the suction side of the blade.
The hanks are then placed in the half-shells in order to be smoothed. The root end walls of the hanks are left practically untouched by the operator. In contrast, the two hanks of each strand are handled by the operator so as to be arranged in pre-established positions along the span of the blade.
Finally, the hanks are worked manually during a smoothing operation in order to cause the material to deform and to present predefined variations in section.
At identified reference positions along the blades, operators make use of templates that embody the sections to be given to the spar so as to ensure that the strands of the hanks are deformed progressively and as regularly as possible.
Performing the third step is found to be difficult.
The shapes of the spar are relatively simple at the fastener bushing and in the running portion of the blade. In simplified manner, the hanks extend substantially vertically in the vicinity of the fastener bushing and substantially horizontally at the pressure side and the suction side. In contrast, the path followed by each hank between those two end zones is complex. This path in particular is twisted in a complex zone referred to as the “connection” zone.
Furthermore, it is not easy to lay the hank fibers coming from the twisted section of the distributed spar so that they are flat on the suction side and the pressure side of the blade.
The twisting in the connection zone can then lead to shapes being poorly reproducible, and also to the mechanical and vibratory characteristics being poorly reproducible from one blade to another.
Furthermore, if filler elements need to be arranged in the blade, then the filler elements need to be complex in shape and to vary from one blade to another when the twisting of the spar is not reproduced identically. Blade fabrication can then be difficult to automate.
Document FR 2 918 347 proposes a fastener bushing enabling the distributed spar to be wound not around a vertical axis but rather around a horizontal axis.
Documents FR 2 321 997, FR 2 030 036, EP 1 035 963, and FR 993 491 are remote from the problems of the invention.
Document FR 2 321 997 discloses a method of fabricating articles of non-circular section by winding continuous filaments.
That method makes use of a stationary mandrel that is inflatable. The mandrel is inflated in order to become circular in shape and is then set into rotation. Continuous filaments are then wound around the mandrel and coated in a curable resin.
The mandrel is then deflated to transform the winding into a deformable sheath.
The deformable sheath is then placed in the cavity of a mold. Thereafter an operator raises the pressure inside the sheath in order to cause the winding to fit closely to the shape of the mold. Finally, the operator causes the resin to cure in order to transform the sheath into a rigid hollow structure.
That method of winding continuous fibers around a stationary mandrel does not appear to be suitable for a distributed spar for a blade.
According to Document FR 2 030 036, an operator performs winding to make a cylindrical element that is to constitute the outer skin of the blade. The cylindrical element is then subdivided into two subassemblies, each of which is placed in a mold.
The spar of the blade is also made by winding.
The spar has a first block of fiber layers presenting an angle of 45° with the longitudinal axis of the blade, and then a second block of fiber layers presenting an angle of 30° with the longitudinal axis, and finally a third block of fiber layers presenting an angle of 15° with the longitudinal axis.
The spar projects to the outside at the blade root through an opening, and a wound attachment is wedged in the opening. Thereafter, the spar is inserted between the two molds in which the two subassemblies of the outer skin of the blade are arranged.
That fabrication method is effective but appears to be difficult to adapt to a distributed spar. Furthermore, multiple steps need to be performed.
Document EP 1 035 963 describes a control system for a fiber-placement device. That document EP 1 035 963 nevertheless does not give any teaching about a spar for a blade.
The fiber-placement method is an alternative to winding a continuous filament.
Fiber placement is a method of laying fibers, tapes, roving, or other similar reinforcement, while dry or when pre-impregnated with resin, on surfaces of various shapes. The placement is performed by using a device having a movable placement head. The placement head thus ensures that the material used is positioned on a surface by making direct mechanical contact with the surface. For this purpose, the placement head is moved by a movement system of the type comprising a robot or a positioning gantry, with degrees of freedom in movement that are servo-controlled depending on the shape of the surface to be covered.
The fiber-placement method thus makes it possible in particular to make surfaces of large dimensions and small thicknesses, within the limits set by the size of the placement head.
In particular, the fiber-placement method makes it possible automatically to perform operations of laying discontinuous fibers by performing starting, laying, and cutting operations.
Document FR 2 993 491 describes a fiber-placement machine.
It should be observed that placing a roving tape by means of a placement system can be difficult. The material in the form of tapes as laid by placement systems tends to be viscous. That method then provides little latitude to an operator for smoothing the tape, and that can appear to be incompatible with making a spar.
Furthermore, the size of the volumes swept by a placement head would not appear to be compatible with the volumes of the half-shells used when fabricating a blade using the above-described method.
Finally, a spar presents large thicknesses, unlike the parts that are generally fabricated by fiber placement.
In addition, a spar presents sections in its span direction that are of a very great variety of kinds. Under such circumstances, fabricating a spar by applying a fiber-placement method would appear to be difficult to perform.
Document U.S. Pat. No. 6,263,936 is also known.